It has long since been recognized that antibodies play an important role in the protection against pathogens not only in the blood but also in fluids that are excreted into the mucosal surfaces of the body. Many pathogens invade the body via the mucosal surfaces. Such surfaces are inherently vulnerable to invasion and colonization by these organisms. Dimeric IgA (dIga) plays an important role in the immune protection of the mucosal and glandular surfaces by blocking entry of the antigens into the mucosal tissues and by neutralizing microbial pathogens (Kaetzel et al., 1991; Mazanec et al., 1992; Mazanec et al., 1993; Lamm, 1997). The pIgR transports polymeric immunoglobulins of the IgA and IgM class, produced by plasma cells within the interstitial space underlying the mucosae, across the epithelial cell layer of the mucosal and glandular surfaces (such as the gastrointestinal tract, respiratory tract, genital tract and mammary gland) into the external secretions (Childers et al., 1989; Kraehenbuhl et al., 1992; Mostov, 1994). The pIgR is important for the immunoglobulin transport into the milk. The pIgR is made by the epithelial cells of the mammary gland. This unique secretory organ produces milk, which not only provides nutritional and metabolic factors to the newborn, but also non-specific and specific antimicrobial factors such as IgA. It is generally accepted that the secretion of non-specific antimicrobial factors such as lactoferrin (Nuijens et al., 1996) and lysozyme and specific antibodies against pathogenic micro-organisms protects not only the mammary gland from infections but also the newborn. The major class of immunoglobulins in mammalian milk is IgA, which originates from B-lymphocytes emerging from mucosal sites such as Peyer's patches of the gut. The level of secretory IgA (S-IgA) depends on the lactation stage (colostrun/milk) and differs among species (Brandtzaeg, 1983; Goldblum et al., 1994).